How do joints promote weathering

Frost heaving is responsible for winter damage to roads all over North America. When salt water seeps into rocks and then evaporates on a hot sunny day, salt crystals grow within cracks and pores in the rock.

The growth of these crystals exerts pressure on the rock and can push grains apart, causing the rock to weaken and break. There are many examples of this on the rocky shorelines of Vancouver Island and the Gulf Islands, where sandstone outcrops are common and salty seawater is readily available Figure 5.

Salt weathering can also occur away from the coast, because most environments have some salt in them. The effects of plants and animals are significant in mechanical weathering. Roots can force their way into even the tiniest cracks, and then they exert tremendous pressure on the rocks as they grow, widening the cracks and breaking the rock Figure 5.

Although animals do not normally burrow through solid rock, they can excavate and remove huge volumes of soil, and thus expose the rock to weathering by other mechanisms. Mechanical weathering is greatly facilitated by erosion, which is the removal of weathering products, allowing for the exposure of more rock for weathering.

Joints form free space in rock by which other agents of chemical or physical weathering can enter. Crystal Growth - As water percolates through fractures and pore spaces it may contain ions that precipitate to form crystals. As these crystals grow they may exert an outward force that can expand or weaken rocks. Thermal Expansion - Although daily heating and cooling of rocks do not seem to have an effect, sudden exposure to high temperature, such as in a forest or grass fire may cause expansion and eventual breakage of rock.

Campfire example. Root Wedging - Plant roots can extend into fractures and grow, causing expansion of the fracture. Growth of plants can break rock - look at the sidewalks of New Orleans for example. Animal Activity - Animals burrowing or moving through cracks can break rock. Frost Wedging - Upon freezing, there is an increase in the volume of the water that's why we use antifreeze in auto engines or why the pipes break in New Orleans during the rare freeze.

As the water freezes it expands and exerts a force on its surroundings. Frost wedging is more prevalent at high altitudes where there may be many freeze-thaw cycles.

Chemical Weathering Since many rocks and minerals are formed under conditions present deep within the Earth, when they arrive near the surface as a result of uplift and erosion, they encounter conditions very different from those under which they originally formed. Minerals that are stable under P, T, H 2 O, and O 2 conditions near the surface are, in order of most stable to least stable:. The main agent responsible for chemical weathering reactions is water and weak acids formed in water.

The most common weak acid that occurs in surface waters is carbonic acid. Carbonic acid is produced in rainwater by reaction of the water with carbon dioxide CO 2 gas in the atmosphere.

Example: Leaching - ions are removed by dissolution into water. Oxidation - Since free oxygen O 2 is more common near the Earth's surface, it may react with minerals to change the oxidation state of an ion. Dehydration - removal of H 2 O or OH - ion from a mineral. Complete Dissolution - all of the mineral is completely dissolved by the water. Living Organisms - Organisms like plants, fungi, lichen, and bacteria can secrete organic acids that can cause dissolution of minerals to extract nutrients.

The role of microorganisms like bacteria has only recent been discovered. For example a granite consisting mostly of quartz is already composed of a mineral that is very stable on the Earth's surface, and will not weather much in comparison to limestone, composed entirely of calcite, which will eventually dissolve completely in a wet climate.

For instance, liquid water can seep into cracks and crevice s in rock. If temperatures drop low enough, the water will freeze. When water freezes, it expand s.

The ice then works as a wedge. It slowly widens the cracks and splits the rock. When ice melts, liquid water performs the act of erosion by carrying away the tiny rock fragments lost in the split. This specific process the freeze-thaw cycle is called frost weathering or cryofracturing.

Temperature changes can also contribute to mechanical weathering in a process called thermal stress. Changes in temperature cause rock to expand with heat and contract with cold. As this happens over and over again, the structure of the rock weakens. Over time, it crumbles. Rocky desert landscapes are particularly vulnerable to thermal stress. The outer layer of desert rocks undergo repeated stress as the temperature changes from day to night.

Eventually, outer layers flake off in thin sheets, a process called exfoliation. Exfoliation contributes to the formation of bornhardt s, one of the most dramatic features in landscapes formed by weathering and erosion. Bornhardts are tall, domed, isolated rocks often found in tropical areas. Sugarloaf Mountain, an iconic landmark in Rio de Janeiro, Brazil, is a bornhardt.

Changes in pressure can also contribute to exfoliation due to weathering. In a process called unloading, overlying materials are removed.

The underlying rocks, released from overlying pressure, can then expand. As the rock surface expands, it becomes vulnerable to fracturing in a process called sheeting.

Another type of mechanical weathering occurs when clay or other materials near rock absorb water. Clay, more porous than rock, can swell with water, weathering the surrounding, harder rock. Salt also works to weather rock in a process called haloclasty. Saltwater sometimes gets into the cracks and pores of rock. If the saltwater evaporate s, salt crystals are left behind. As the crystal s grow, they put pressure on the rock, slowly breaking it apart. Soils retain rainwater so that rocks covered by soil are subjected to chemical reactions with water much longer than rocks not covered by soil.

Soils are also host to a variety of vegetation, bacteria and organisms that produce an acidic environment which also promotes chemical weathering. Minerals in a rock buried in soil will therefore break down more rapidly than minerals in a rock that is exposed to air. The longer a rock is exposed to the agents of weathering, the greater the degree of alteration, dissolution and physical breakup.

Lava flows that are quickly buried by subsequent lava flows are less likely to be weathered than a flow which remains exposed to the elements for long periods of time. Chemical weathering is a process where minerals in a rock may be converted into clays, oxidized or simply dissolved.

Silicates comprise almost all minerals in igneous rocks and are also important components in metamorphic rocks. Not all silicates, however, survive weathering processes to become incorporated into sedimentary rocks. Figure 6. For example, interlocking silicate grains in fresh granite gradually decay along crystal boundaries due to conversion to clays. Eventually cracks open around the boundaries, the rock weakens and easily disintegrates.

Water dissolves some of the solid, leaving behind an altered material and producing a solution containing substances drawn from the original solid coffee grounds. The acid rainwater than reacts with minerals on the exposed rock face.

Reaction of silicates with carbonic acid and water produces clays and also releases Si and certain cations into water as dissolved constituents:. The dissolved cations are carried away by rain and river waters and ultimately transported to the oceans.

In tropical regions, clays can further react with water to form Bauxite Al-hydroxide , an ore which is a major source of Al. Slightly acidic rainwater can also react with non-silicates in a rock or soil. For instance, carbonic acid can dissolve carbonates such as calcite so that all constinuents go into solution.